During production of oil and gas from a subterranean reservoir, the well stream will normally contain oil, gas, water and some solid particles. In order to separate the various fluids and solids, a dedicated process system for the well stream is constructed. The separation is made in several stages, where gravity forces alone, and the “fine separation” carry out the “bulk separation” of the various phases or purification is mainly utilising centrifugal forces and inertial forces together with the gravity force. A major challenge appearing in most separation stages is to remove liquid droplets from a gas stream where the liquid content in the gas is low, typically less than 1 vol % of the total volumetric flow. It is still important to immediately remove most of this liquid in order to protect downstream equipment such as compressors and additional dewatering equipment. This is done in large separators, horizontally or vertically oriented. In the following, separators dedicated to separate gas/liquid mixtures containing less than said 2 vol % liquid are denoted gas scrubbers.
In gas scrubbers, the separation takes place in several stages. First, the gas enters through an inlet nozzle, which—for vertical oriented scrubbers—may be located approximately at the middle of the scrubber in its vertical direction. At the inlet nozzle a momentum breaker plate, a vane diffuser or any similar device may be arranged in order to distribute fluids across the separator cross-sectional area. Already here, the largest drops are separated and fall down onto the liquid reservoir in the lower part of the separator.
The gas then flows upwards into a calm zone, or deposition zone, where further droplets due to gravity fall down onto the liquid surface below, alternatively first deposit on the separator wall and thereafter drain downwards on the wall.
Close to the gas outlet at the top of the separator, the gas is forced to flow through a number of parallelly mounted demisting cyclones, or some other known droplet removal equipment. The liquid that is separated by the mist eliminators is guided down to the liquid reservoir through a pipe called a drainpipe, which preferably could be submerged in the liquid reservoir. The pressure drop across the mist eliminators is an important dimensioning parameter when selecting droplet removal equipment. A large pressure drop causes an equally large suction in the drainpipe; hence the liquid level in the drainpipe is higher situated from datum than the liquid level of the reservoir. If the pressure drop is too large, liquid will be sucked up from the reservoir through the drainpipe and into the gas instead of draining down trough this.
Two main types of demisting cyclones are known. One type is characterized by a reversal of the direction of the gas in axial direction, and is here denoted as reversible flow cyclone while the other type is characterized by having the same axial direction of the flow denoted as axial flow cyclone. These two main types are further mentioned with reference to the drawings. The reversible cyclone is known to have a good ability to separate droplets from gas even at high liquid loading, but is also known to have problematically high pressure drop. The axial flow cyclone has considerably less pressure drop, but does not possess the same good ability of separating liquid droplets at high liquid loading.
Examples of known mist eliminators can be found in the patent applications PCT/NL97/00350, PCT/NL99/00677 and NL 20001016114. These are commented in more detail with reference to the drawings.